Good question and this is certainly something someone new to the space would struggle with. The reasons are different, but an alien new to our planet could land and ask "Why do you have so many fiat currencies? Why doesn't everyone share a currency?" The current state of matters is what everyone knows, whereas those new to the space see it as a barrier to entry due to a knowledge gap.

Obviously each fiat currency has more to do with governments, zones, or countries with the ability to standardize and/or control its value. Whereas crypto has different options that try to solve technology gaps and can be used as an incentive to use/build the product/network.